Steel laminations have been used for decades in low frequency magnetic components. The design of stacked magnetic components must take into account that the magnetic flux is confined in planes parallel to the sheet surfaces. Additionally, there are difficulties with miniaturisation and waste material with steel laminations, which can be important for some type of electric motors.
The idea to use iron powder in magnetic components was first introduced by Fritts and Heaviside in the 1880's. Since the beginning of the century, iron powder has been used for the production of magnetic components (iron powder cores were introduced in the U.S. to replace wire cores around 1915). Powder metallurgy offers the possibility of controlling the spatial distribution of the magnetic flux and allows practically full utilization of materials even for the manufacture of complicated shapes. Recent advances in powder metallurgy offer new opportunities in the design of electromagnetic components. Several authors have shown the advantages to use iron/resin composites especially for applications in the medium and high frequency ranges.
When a magnetic material is exposed to an alternating magnetic field, it dissipates energy. The power dissipated under an alternating field is defined as core losses. The core losses are mainly composed of hysteresis and eddy current losses. Hysteresis losses are due to the energy dissipated by the domain wall movement. The hysteresis losses are proportional to the frequency and are mainly influenced by the chemical composition and the structure of the material.
Eddy currents are induced when a magnetic material is exposed to an alternating magnetic field. These currents lead to an energy loss through Joule (resistance) heating. Eddy current losses are expected to vary with the square of the frequency, and inversely with the resistivity. The relative importance of the eddy current losses thus depends on the electrical resistivity of the material.
Sintered iron powder components are currently used to make parts for DC magnetic applications. However, sintered parts have low resistivity and are generally not used in AC applications. For applications in alternating magnetic field (AC), a minimal threshold resistivity is required and powder mixes containing insulating resins are generally used. The resin is used to insulate and bind the magnetic particles together. It is well known that iron-resin composites have very low eddy current losses and perform well at moderate and high frequency, while eddy current losses are important at those frequencies in stack assemblies. However, at low frequencies, e.g., 60 Hz, the eddy current portion of the losses is not as important in stack assemblies and the performance of the iron-resin composites is limited by their hysteresis losses. In fact, the hysteresis portion of the losses is higher in iron-resin composite than in stack assemblies. During the fabrication of soft magnetic components with iron powders, stresses are induced in the material. These stresses significantly increase the hysteresis portion of the losses. These stresses can be relaxed by heating the component at high temperature. However, the resin generally used in iron-resin composites cannot withstand the temperature used to relax the stresses. After the thermal treatment, the parts generally do not have sufficient mechanical strength and electrical resistivity for many applications.
Powder formulations for the fabrication of annealed soft magnetic components for AC soft magnetic applications have been described in patent literature.
U.S. Pat. No. 5,595,609 issued Jan. 21, 1997 to Gay discloses polymer-bonded soft magnetic body that can be annealed at temperature around 500.degree. C. The magnetic powder used is encapsulated with a thermoplastic coating selected from the group of polybenzimidazole and polyimides having heat deflection temperatures of at least about 400.degree. C.
U.S. Pat. No. 5,754,936 issued May 19, 1998 to Jansson, and WO 95/29490 disclose phosphate coated powders that can be used for the fabrication of annealed components. After compaction, the components are treated at temperature ranging from 350 to 500.degree. C. to stress relief the magnetic powders.
U.S. Pat. No. 5,352,522 issued Oct. 4, 1994 to Kugimiya et al. discloses oxide coated powders that can be processed at high temperature (800.degree. C.) for the fabrication of soft magnetic components.
European patent application EPO 088 992 A2 discloses oxide coated powders for the fabrication of magnetic components processed at high temperatures (900.degree. C.).
U.S. Pat. No. 4,601,765 issued Jul. 22, 1986 to Soileau et al. discloses silicate coatings for the fabrication of annealed components.
F. Hanejko et al. "Application of High Performance Material Processing Electromagnetic Products" in the Proceedings of the 1998 International Conference on Powder Metallurgy & Particulate Materials, May 31-June 4th, Las Vegas, Nev., 1998, p. 8-13. presented results on annealed soft magnetic components fabricated with coated powders.
The above-discussed prior art discloses coated powders for the fabrication of annealed soft magnetic components for AC soft magnetic applications. Coating the powder represents an additional step during the preparation of the material. It involves additional cost and the preparation of the powder may require additional equipment. None of the prior art discloses compositions produced with uncoated powders. In addition, in most of the prior art processes, the composition does not contain an admixed lubricant and cannot be processed using simple compaction at room temperature without using die wall lubrication.
Other references of interest are: R. W. Ward and D. E. Gay, "Composite Iron Material", U.S. Pat. No. 5,211,896 (1993); H. Rutz and F. G. Hanejko, "Doubly-Coated Iron Particles", U.S. Pat. No. 5,063,011 (1991); G.Katz, "Powdered Iron Magnetic Core Materials", U.S. Pat. No. 2,783,208 (1957); and P. N. Roseby, "Magnet Core", U.S. Pat. No. 1,789,477 (1931). This prior art does not refer to thermal treated materials having reduced hysteresis losses.
Mixes (compositions) composed of iron powder and lubricant have been used for a long time for powder metallurgy applications. The lubricant is used to ease the compaction of the powder, ease the ejection of the part from the die and to minimize die wear. After compaction, the part does not have sufficient mechanical properties and must be sintered to create metallurgical bonds between the particles. Sintering is generally done at temperature ranging from 1000.degree. C. up to 1200.degree. C. Specimens compacted from iron-lubricant mixtures cannot be used in the green (non-heated) nor the sintered state for the fabrication of components for AC soft magnetic applications, having low core losses. The green parts do not have sufficient mechanical strength while the sintered components do not have sufficient electrical resistivity to maintain low eddy current losses.
It is an object of the present invention to provide powder compositions and a process for the fabrication of soft magnetic components intended for low frequency soft magnetic applications.
It is a further object of this invention to increase the mechanical strength of the components without sintering.